One example of conventional spectral filters is disclosed in Patent Document 1 identified below. The spectral filter of Patent Document 1 includes a first interference filter and a second interference filter. The first interference filter, of which the cutoff wavelength WL lengthens monotonically in one direction, transmits light in a wavelength range longer than the cutoff wavelength WL. The second interference filter, of which the cutoff wavelength WS lengthens monotonically in one direction, transmits light in a wavelength range shorter than the cutoff wavelength WS. An interference filter of which the cutoff wavelength varies monotonically in one direction as just mentioned can be configured as a so-called wedge-form interference filter of which the film thickness increases continuously in one direction. In Patent Document 1, the first and second interference filters are given equal film thickness gradients.
The first and second interference filters are laid over each other such that the directions in which the cutoff wavelengths WL and WS increase monotonically coincide and in addition that the cutoff wavelength WL of the first interference filter is shorter than the cutoff wavelength WS of the second interference filter at any mutually corresponding positions. With this configuration, at any position in the one direction, the cutoff characteristic on each of the longer- and shorter-wavelength sides of the peak wavelength of the spectral transmittance can be set with a separate interference filter. It is thus possible to easily obtain spectral characteristics superior to those obtained with a single interference filter.